Message-ID: <20150218164732.GH32600@odux.rfo.atmel.com>
Date:	Wed, 18 Feb 2015 17:47:32 +0100
From:	Ludovic Desroches <ludovic.desroches@...el.com>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
CC:	Ludovic Desroches <ludovic.desroches@...el.com>,
	Mark Rutland <mark.rutland@....com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Guenter Roeck <linux@...ck-us.net>,
	Rob Herring <robherring2@...il.com>,
	Tony Lindgren <tony@...mide.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/4] of: DT quirks infrastructure

On Wed, Feb 18, 2015 at 06:39:01PM +0200, Pantelis Antoniou wrote:
> Hi Ludovic,
> 
> > On Feb 18, 2015, at 18:32 , Ludovic Desroches <ludovic.desroches@...el.com> wrote:
> > 
> > Hi,
> > 
> > Great something we are waiting for a long time!
> > 
> > On Wed, Feb 18, 2015 at 05:53:50PM +0200, Pantelis Antoniou wrote:
> >> Hi Mark,
> >> 
> >>> On Feb 18, 2015, at 17:41 , Mark Rutland <mark.rutland@....com> wrote:
> >>> 
> >>> Hi Pantelis,
> >>> 
> >>> On Wed, Feb 18, 2015 at 02:59:34PM +0000, Pantelis Antoniou wrote:
> >>>> Implement a method of applying DT quirks early in the boot sequence.
> >>>> 
> >>>> A DT quirk is a subtree of the boot DT that can be applied to
> >>>> a target in the base DT resulting in a modification of the live
> >>>> tree. The format of the quirk nodes is that of a device tree overlay.
> >>>> 
> >>>> For details please refer to Documentation/devicetree/quirks.txt
> >>>> 
> >>>> Signed-off-by: Pantelis Antoniou <pantelis.antoniou@...sulko.com>
> >>>> ---
> >>>> Documentation/devicetree/quirks.txt | 101 ++++++++++
> >>>> drivers/of/dynamic.c                | 358 ++++++++++++++++++++++++++++++++++++
> >>>> include/linux/of.h                  |  16 ++
> >>>> 3 files changed, 475 insertions(+)
> >>>> create mode 100644 Documentation/devicetree/quirks.txt
> >>>> 
> >>>> diff --git a/Documentation/devicetree/quirks.txt b/Documentation/devicetree/quirks.txt
> >>>> new file mode 100644
> >>>> index 0000000..789319a
> >>>> --- /dev/null
> >>>> +++ b/Documentation/devicetree/quirks.txt
> >>>> @@ -0,0 +1,101 @@
> >>>> +A Device Tree quirk is the way which allows modification of the
> >>>> +boot device tree under the control of a per-platform specific method.
> >>>> +
> >>>> +Take for instance the case of a board family that comprises of a
> >>>> +number of different board revisions, all being incremental changes
> >>>> +after an initial release.
> >>>> +
> >>>> +Since all board revisions must be supported via a single software image
> >>>> +the only way to support this scheme is by having a different DTB for each
> >>>> +revision with the bootloader selecting which one to use at boot time.
> >>> 
> >>> Not necessarily at boot time. The boards don't have to run the exact
> >>> same FW/bootloader binary, so the relevant DTB could be programmed onto
> >>> each board.
> >>> 
> >> 
> >> That has not been the case in any kind of board I’ve worked with.
> >> 
> >> A special firmware image that requires a different programming step at
> >> the factory to select the correct DTB for each is always one more thing
> >> that can go wrong.
> >> 
> > 
> > I agree. We have boards with several display modules, even if it seems
> > quite easy to know which dtb has to be loaded since we use a prefix
> > describing the display module (_pda4, _pda7, etc.) it is a pain for
> > customers. Moreover you can add the revision of the board, we have a
> > mother board and a cpu module so it can quickly lead to something like
> > this:
> > at91-sama5d31ek_mb-revc_cm-revd_pda7.
> > 
> > It is only an example, at the moment it is a bit less complicated but I
> > am not so far from the reality: sama5d31ek_revc_pda7.dts,
> > sama5d33ek_revc_pda4.dts, etc. For a SoC family, we have 27 DTS files…
> > 
> 
> I bet it’s easy to screw up no? Any horror stories from the factory floor? :)

Yes it is.

I keep these horror stories for Halloween! At the moment no incident because
we are trying hard to find workarounds with the help of our
In-system programmer or U-boot but the goal is to not rely on a particular
component to do this job.

> 
> > As for the single zImage, we should find a way to have a single DTB.
> > 
> >>>> +While this may in theory work, in practice it is very cumbersome
> >>>> +for the following reasons:
> >>>> +
> >>>> +1. The act of selecting a different boot device tree blob requires
> >>>> +a reasonably advanced bootloader with some kind of configuration or
> >>>> +scripting capabilities. Sadly this is not the case many times, the
> >>>> +bootloader is extremely dumb and can only use a single dt blob.
> >>> 
> >>> You can have several bootloader builds, or even a single build with
> >>> something like appended DTB to get an appropriate DTB if the same binary
> >>> will otherwise work across all variants of a board.
> >>> 
> >> 
> >> No, the same DTB will not work across all the variants of a board.
> >> 
> >>> So it's not necessarily true that you need a complex bootloader.
> >>> 
> >> 
> >>>> +2. On many instances boot time is extremely critical; in some cases
> >>>> +there are hard requirements like having working video feeds in under
> >>>> +2 seconds from power-up. This leaves an extremely small time budget for
> >>>> +boot-up, as low as 500ms to kernel entry. The sanest way to get there
> >>>> +is by removing the standard bootloader from the normal boot sequence
> >>>> +altogether by having a very small boot shim that loads the kernel and
> >>>> +immediately jumps to kernel, like falcon-boot mode in u-boot does.
> >>> 
> >>> Given my previous comments above I don't see why this is relevant.
> >>> You're already passing _some_ DTB here, so if you can organise for the
> >>> board to statically provide a sane DTB that's fine, or you can resort to
> >>> appended DTB if it's not possible to update the board configuration.
> >>> 
> >> 
> >> You’re missing the point. I can’t use the same DTB for each revision of the
> >> board. Each board is similar but it’s not identical.
> >> 
> >>>> +3. Having different DTBs/DTSs for different board revisions easily leads to
> >>>> +drift between versions. Since no developer is expected to have every single
> >>>> +board revision at hand, things are easy to get out of sync, with board versions
> >>>> +failing to boot even though the kernel is up to date.
> >>> 
> >>> I'm not sure I follow. Surely if you don't have every board revision at
> >>> hand you can't test quirks exhaustively either?
> >>> 
> >> 
> >> It’s one less thing to worry about. For example in the current mainline kernel
> >> already there is a drift between the beaglebone white and the beaglebone black.
> >> 
> >> Having the same DTS is just easier to keep things in sync.
> >> 
> >>> Additionally you face the problem that two boards of the same variant
> >>> could have different base DTBs that you would need to test that each
> >>> board's quirks worked for a range of base DTBs.
> >>> 
> >> 
> >> This is not a valid case. This patch is about boards that have the same base DTB.
> >> 
> >>>> +4. One final problem is the static way that device tree works.
> >>>> +For example it might be desirable for various boards to have a way to
> >>>> +selectively configure the boot device tree, possibly by the use of command
> >>>> +line options.  For instance a device might be disabled if a given command line
> >>>> +option is present, different configuration to various devices for debugging
> >>>> +purposes can be selected and so on. Currently the only way to do so is by
> >>>> +recompiling the DTS and installing, which is an chore for developers and
> >>>> +a completely unreasonable expectation from end-users.
> >>> 
> >>> I'm not sure I follow here.
> >>> 
> >>> Which devices do you envisage this being the case for?
> >>> 
> >>> Outside of debug scenarios when would you envisage we do this?
> >>> 
> >> 
> >> We already have to do this on the beaglebone black. The onboard EMMC and HDMI
> >> devices conflict with any capes that use the same pins. So you have to
> >> have a way to disable them so that the attached cape will work.
> >> 
> >>> For the debug case it seems reasonable to have command line parameters
> >>> to get the kernel to do what we want. Just because there's a device in
> >>> the DTB that's useful in a debug scenario doesn't mean we have to use it
> >>> by default.
> >> 
> >> I don’t follow. Users need this functionality to work. I.e. pass a command
> >> line option to use different OPPs etc. Real world usage is messy.
> >> 
> >>> 
> >>>> +Device Tree quirks solve all those problems by having an in-kernel interface
> >>>> +which per-board/platform method can use to selectively modify the device tree
> >>>> +right after unflattening.
> >>>> +
> >>>> +A DT quirk is a subtree of the boot DT that can be applied to
> >>>> +a target in the base DT resulting in a modification of the live
> >>>> +tree. The format of the quirk nodes is that of a device tree overlay.
> >>>> +
> >>>> +As an example the following DTS contains a quirk.
> >>>> +
> >>>> +/ {
> >>>> +       foo: foo-node {
> >>>> +               bar = <10>;
> >>>> +       };
> >>>> +
> >>>> +       select-quirk = <&quirk>;
> >>>> +
> >>>> +       quirk: quirk {
> >>>> +               fragment@0 {
> >>>> +                       target = <&foo>;
> >>>> +                       __overlay {
> >>>> +                               bar = <0xf00>;
> >>>> +                               baz = <11>;
> >>>> +                       };
> >>>> +               };
> >>>> +       };
> >>>> +};
> >>>> +
> >>>> +The quirk when applied would result at the following tree:
> >>>> +
> >>>> +/ {
> >>>> +       foo: foo-node {
> >>>> +               bar = <0xf00>;
> >>>> +               baz = <11>;
> >>>> +       };
> >>>> +
> >>>> +       select-quirk = <&quirk>;
> >>>> +
> >>>> +       quirk: quirk {
> >>>> +               fragment@0 {
> >>>> +                       target = <&foo>;
> >>>> +                       __overlay {
> >>>> +                               bar = <0xf00>;
> >>>> +                               baz = <11>;
> >>>> +                       };
> >>>> +               };
> >>>> +       };
> >>>> +
> >>>> +};
> >>>> +
> >>>> +The two public method used to accomplish this are of_quirk_apply_by_node()
> >>>> +and of_quirk_apply_by_phandle();
> >>>> +
> >>>> +To apply the quirk, a per-platform method can retrieve the phandle from the
> >>>> +select-quirk property and pass it to the of_quirk_apply_by_phandle() node.
> >>>> +
> >>>> +The method which the per-platform method is using to select the quirk to apply
> >>>> +is out of the scope of the DT quirk definition, but possible methods include
> >>>> +and are not limited to: revision encoding in a GPIO input range, board id
> >>>> +located in external or internal EEPROM or flash, DMI board ids, etc.
> >>> 
> >>> It seems rather unfortunate that to get a useful device tree we have to
> >>> resort to board-specific mechanisms. That means yet more platform code,
> >>> which is rather unfortunate. This would also require any other DT users
> >>> to understand and potentially have to sanitize any quirks (e.g. in the
> >>> case of Xen).
> >> 
> >> The original internal version of the patches used early platform devices and
> >> a generic firmware quirk mechanism, but I was directed to the per-platform
> >> method instead. It is perfectly doable to go back at the original implementation
> >> but I need to get the ball rolling with a discussion about the internals.
> > 
> > I also think we should used early platform devices to not add platform
> > specific code. What were the cons to swith to per-platform method?
> > 
> 
> Supposedly easier to accept. /me shrugs
> 
> >> 
> >>> 
> >>> Mark.
> >> 
> >> Regards
> >> 
> >> — Pantelis
> >> 
> > 
> > Regards
> > 
> > Ludovic
> 
> Regards
> 
> — Pantelis
> 
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